There are production fluids, produced from oil reservoirs, which do not lend themselves to being efficiently pumped on a long term basis with commercial downhole pumps. One such fluid is the viscous, gassy, sand-laden fluid produced from a sand reservoir undergoing in situ combustion. When a conventional downhole pump is newly installed in this service, it often loses efficiency or becomes gas-locked due to the presence of gassy emulsions.
This problem can be better comprehended when discussed in the context of a prior art downhole pump illustrated in simplified form in FIGS. 1a and 1b. The known pump comprises a barrel a which seats in the seating nipple b of a tubing string c. A rod string d extends from surface into the barrel a. The rod string d carries a hollow cylindrical plunger f at its lower end. The plunger f has a small clearance fit (in the order of 1/1000 of an inch) with respect to the barrel a. This close fit is relied on to provide a liquid-tight seal between them. A ball valve member r and ring seat member s, forming the functioning parts of a travelling valve g, are positioned at the base of the plunger f; the travelling Valve g is operative to permit produced wellbore fluid to enter the plunger f on the down stroke of the rod string; on the up stroke, the valve g closes or seals the inlet x of the plunger f. Ports h connect the bore i, extending through the plunger f, with the barrel chamber j. Oil transfer ports k, extending through the barrel wall, connect the barrel chamber j with the tubing annulus 1 at a point above the seating nipple b. Flow through the oil transfer ports k is controlled by a standing valve m, comprising a ring valve member n and a stationary ring seat member o. The ring valve member n has a small clearance fit (in the order of 1/1000 of an inch) around the rod string d to effect a liquid-tight seal.
A comparatively high pressure, generated by the long column of fluid in the tubing annulus l, exists at the oil transfer ports k; a comparatively low pressure, generated by the short column of fluid in the casing annulus p, exists at the base of the plunger travelling valve g.
The operation of this prior art downhole pump is now described. When the plunger f and rod string d are on the down stroke (as shown in FIG. 1b), the standing valve ring valve member n is seated, thereby isolating the tubing annulus 1 from the barrel chamber j by closing the oil transfer ports k. The high pressure present in the tubing annulus l is therefore sealed off from the barrel chamber j. The downward movement of the plunger f creates a low pressure condition in the barrel chamber j. Under these conditions, the relatively higher pressure present in the casing annulus p is able to unseat the ball valve member r of the travelling valve g, and thus reservoir fluid is able to enter the plunger f and fill the barrel chamber j. Upon completion of the downstroke, the pressure in the barrel chamber j equalizes with the casing annulus p pressure and the travelling valve g closes. As the plunger f moves upward, it compresses the fluid in the barrel chamber j until this pressure exceeds that of the tubing annulus 1 and the standing valve ring valve member n is unseated, as shown in FIG. 1a. The plunger f now acts as a swab and pushes the fluid within the barrel chamber j through the oil transfer port k into the tubing annulus 1. When the upstroke is completed and the fluid has been so displaced, the rod string d begins to move downwardly, thereby seating the ring valve member n, and the cycle is repeated.
These prior art downhole pumps are affected by a problem, referred to as "gas-locking". As previously mentioned, the fluid, produced by a reservoir undergoing in situ combustion, contains considerable quantities of combustion gases. Gas-locking occurs when there is so much gas in the fluid in the plunger bore and barrel chamber, that the plunger simply compresses this gas on the upstroke, rather than displacing fluid (that is, liquid and gas) into the tubing annulus. The result of gas-locking is that little, if any fluid is removed by the pump, since the gas compresses on the upstroke, so that the pressure in the barrel chamber does not become high enough to open the standing valve, and the gas expands on the downstroke, so that the pressure does not become low enough for the travelling valve to open.
There is thus a need for an improved system which counteracts gas-locking of the pump.